Offshore structures must be capable of withstanding forces from the ocean environment over the entire life of the structure. Wind, waves and currents are the principle sources of dynamic loading. Currents and wind are known for inducing structures into a vibratory motion, often referred to as Vortex Induced Vibration (VIV). This vibration is the result of the fluid (e.g. water and/or air) shedding in an alternating fashion from opposite sides of the structure. This alternating shedding of vortices creates an oscillatory pressure field in the fluid around the structure. Depending on the resistance of the structure, these forces can be large enough to induce movement of the structure. The rate of this shedding depends on the size of the structure and the speed of the fluid. As the shedding frequency approaches a natural vibration frequency of the structure, the oscillatory pressures can induce a resonant vibration on the structure. VIV places demands on strength and fatigue resistance of offshore structures.
Interest in VIV-induced motions of offshore structures and tow tanks with sufficient capability to test at the high Reynolds numbers necessary to be applicable for offshore applications have been present for more than twenty-five years. Yet, accurate prediction of full-scale behavior of offshore systems has remained elusive due to the inherent limitations in testing and analysis procedures used in the field to date.
The presently known methodologies for testing offshore structures for VIV motions may generally be classified into three categories. First there is testing in current flumes where the current flows past the test body. However for open channel testing, only low Reynolds numbers may be achieved and there is little control over the turbulence intensity level. For cavitation channel testing, which operates at higher than atmospheric pressure, the current design and implementation of test rigs do not allow for large amplitude oscillation at wide ranges of flow velocity.
Next, there is testing in tow tanks where the test body is towed through a long tank. This type of testing is normally hampered by mechanical damping effects resulting from the use of a test rig that holds the test body. Such mechanical damping effects are either ignored or improperly quantified in subsequent data processing. Furthermore, some testing is accomplished with a vertical test body which pierces the surface of the water fluid body. Tests where the test body is oriented vertically and pierces the water surface can generate waves and be unacceptable for high Reynolds number testing.
Then there is also forced oscillation testing methods where the test body is forced to oscillate at given frequencies and amplitudes. At high Reynolds numbers, the hydrodynamic force is large and is dominated by inertia effects that tend to overwhelm lift and damping forces. The relatively large inertia force coupled with components orders of magnitude smaller render measurement of the smaller forces difficult and inaccurate.
To summarize, available laboratory test set-ups and procedures have also been unable to consistently reproduce full scale observed behavior. Problems inherent to existing test rigs include the inability to properly account for mechanical damping in the test set-up, the inability to model free-surface wave effects and the inability to properly model turbulence. In addition, analytical methods for predicting VIV for the range of Reynolds number and complex geometries associated with typical offshore structures have proven to be inadequate. Embodiments of the VIV testing apparatus and procedures outlined herein are capable of reproducing observed full scale phenomena and provide a major improvement compared to other available test rigs and test methodologies.